1. Field of the Invention
The invention relates to a method for the crystallization of a substantially water-insoluble aromatic dicarboxylic acid to obtain large easily-handled crystals of high purity. The method is particularly applicable to the extremely water-insoluble aromatic dicarboxylic acid, 2,6-naphthalenedicarboxylic acid, which is difficult otherwise to purify and crystallize.
2. Brief Description of the Prior Art
Aromatic dicarboxylic acids are major industrial intermediates for the preparation of condensation polymers, especially thermoplastic polyesters and polyamides. These are used in films, fibers, machine parts, electrical parts, containers, and the like. For these applications, it is important that the aromatic dicarboxylic acid be made available to the polymer manufacturer in high purity, free of water or other solvents, and in easily-handled, non-dusty, non-sticky form. Purification by crystallization would seem to be a reasonable way to accomplish these requirements, but crystallization of the very water-insoluble aromatic dicarboxylic acids has been difficult and challenging. A particularly difficult case because of its extreme water insolubility is 2,6-naphthalenedicarboxylic acid.
2,6-Naphthalenedicarboxylic acid (hereinafter called 2,6-NDA) is a product which can be made by oxidation of 2,6-di-lower-alkylnaphthalene, or by disproportionation of naphthalenecarboxylic acid salts, said precursors having been made by oxidation of monoalkylnaphthalenes. Commercially feasible routes are known which employ molecular oxygen as the oxidant.
2,6-NDA is a useful intermediate for making condensation polymers of the polyester and polyamide types, having advantageous properties. For example, linear polyesters and polyamides made from 2,6-NDA, in comparison to analogous polymers made from terephthalic acid, can have superior thermal, mechanical, optical, and electrical properties.
These advantages of 2,6-NDA in linear condensation polymers are best achieved by use of 2,6-NDA in a high state of purity. Impurities such as the monoaldehyde monocarboxylic acid analog of 2,6-NDA act as chain ends and limit the achievable molecular weight, whereas impurities such as benzenetricarboxylic acids cause chain branching and limit the linearity, with generally adverse effects on polymer mechanical properties. Aldehyde and metal impurities also have adverse effects on color, clarity, electrical properties, and processability. Therefore, it is important to have effective and economical processes for purification of 2,6-NDA to remove or substantially reduce these and other impurities which otherwise will be found in industrially produced 2,6-NDA.
Because of the difficulty of crystallizing 2,6-NDA, many process designers have avoided doing so and instead have chosen to conduct the purification step on an ester or a salt of 2,6-NDA and then subsequently convert said ester or salt back to 2,6NDA; for example, Norton (to Sun Ventures Inc.), U.S. Pat. No. 3,965,160 (Jun. 22, 1976). In this patent, air oxidation is used while the 2,6-NDA is in the form of a salt, to remove metals. Teijin Ltd., in Japanese Kokai 50/160248 (Dec. 25, 1975) [Chem. Abs. 84:150408] describe purifying the monopotassium salt of 2,6-NDA and then converting back to 2,6-NDA by adding hydrochloric acid.
Other proposed processes involve use of organic solvents to extract impurities from 2,6-NDA. Norton, Ger. Appl. 2331251 (January 1974) [Chem. Abs. 80: 146601] teaches use of acetic acid and toluene or xylene, and Norton in Ger. Appl. 2400964 (July 1974) [Chem. Abs. 81: 120233] teaches the use of xylene, ethylbenzene or tetralin. Kulakov et al., Russian Patent 486008 (September 1975) [Chem. Abs. 83: 206016] teaches the use of aliphatic acids as solvents for purifying 2,6-NDA. Crystallization of 2,6-NDA from its own melt is impracticable because the compound melts above 300 degrees C with decomposition.
The processes which involve the use of a solvent entail solvent losses and/or the cost of solvent recovery, and the risk of air pollution by solvent vapors. It is also difficult to assure complete removal of solvent from the product.
Processes which require esterification, purification of the ester, and hydrolysis back to acid inherently require two extra chemical steps with inherent costs and usage of reagents.
Processes, such as the Teijin process mentioned above, which involve purification of the salt in aqueous solution followed by reacidification and recovery of 2,6-NDA can be effective in removing certain of the impurities but they have a non-obvious problem which imposes a serious difficulty. When 2,6-NDA is precipitated from its salts by addition of an acid such as hydrochloric, sulfuric or acetic, the 2,6-NDA forms a very fine suspension, in which the small particles appear to be of amorphous or microcrystalline nature. When this suspension is centrifuged or filtered out, the filter cake will contain large amounts of the aqueous solution (the "mother liquor") up to 70 percent. A filter cake of this sort, containing a large amount of the mother liquor, is hard to suck dry and hard to wash free of impurities. When such filter cakes are dried, the product 2,6-NDA is a very light dusty and electrostatically charged powder which is very difficult to handle, and not very pure. In this dusty form, it can present a fire and inhalation hazard. None of the prior art purification schemes address this problem of obtaining easily handled non-dusty non-sticky large crystals.
Simple recrystallization of 2,6-NDA from water fails because of the low solubility of 2,6-NDA even in very hot water, less than 0.1% at 100 deg. C. Digestion of the small amorphous or microcrystalline particles of 2,6-NDA in hot water fails to bring about redeposition to the desired large well-formed rapid-settling crystals. The crystal growth process known as "ripening" or "Ostwald ripening", where a slurry of small crystals is converted to a rapid settling dispersion of large crystals by aging the slurry is found to fail when applied to 2,6-NDA.
It is an object of the present invention to make available a convenient and effective process for making purified 2,6-NDA from relatively impure 2,6-NDA without the use of organic solvents, without the need for esterification and hydrolysis, and with the formation of rapid-settling, easily-filtered, easily-dried, non-sticky, non-dusty large crystals of 2,6-NDA. It is a further object of the invention to make available a purification process for 2,6-NDA which permits removal of undesirable impurities and formation of easily-handled crystals.
It is a further object of the invention to make available a method for preparing purified well-formed crystals of other substantially water-insoluble aromatic dicarboxylic acids. By substantially water-insoluble is meant a solubility of less than 0.1% in water at 100 degrees C. Examples of such acids include the other isomers of naphthalenedicarboxylic acid, as well as 4,4'-oxydibenzoic acid, and 1,1,3-trimethyl-3-phenylindan-4',5-dicarboxylic acid.